In a typical piezoelectric apparatus, a piezoelectric element is mechanically connected to another part, such as a mechanical resonator, to transmit vibratory motion into that part when an appropriate electrical signal is applied to the piezoelectric element. Such a piezoelectric apparatus generally falls into one of two categories. In the first category, the piezoelectric element is placed in static compression along the principal axis of piezoelectric action by a mechanical preload in order to prevent potentially damaging tensile forces on the piezoelectric element during operation of the apparatus. In the second category, such a preload is not necessary. An apparatus comprising a multilayer, stack-type piezoelectric element typically falls into the first category, while an apparatus comprising a bending-type or shear-type piezoelectric element typically falls into the second category. This invention relates to an apparatus of the first category and methods for manufacturing the same.
A variety of methods are known in the prior art to place a piezoelectric element in static compression. A first, often-used method comprises placing the piezoelectric element between two rigid surfaces and urging the surfaces together using, for example, one or more bolts that extend through holes in those surfaces. The need for assembling the one or more bolts is a drawback of the method and apparatus as this assembly step and required hardware typically increases manufacturing costs.
In a second method, the part that is to be mechanically connected to the piezoelectric element comprises an opening. The part could, for example, serve as a mechanical resonator. The dimension of the piezoelectric element along the principal axis of piezoelectric action is selected somewhat larger than the corresponding dimension of the opening in the resonator. In order to accommodate the piezoelectric element, some of the resonator material surrounding the opening must extend and deform elastically, or even plastically. U.S. Pat. No. 6,664,714 teaches this method. As a result of the deformation, the piezoelectric element is held in compression within the opening by the resonator material surrounding the opening. The need for maintaining certain mechanical tolerances for all the parts involved is a drawback of this method. Also, the process of inserting the piezoelectric element into the opening can pose difficulties as it may lead to scraping or abrasion on both the piezoelectric element and the inside of the opening. Also, the forces necessary to insert the piezoelectric element may damage it. Furthermore, the resonator is separately manufactured prior to the step of inserting the piezoelectric element.
Metals are typically used to form the part holding a piezoelectric element in compression. In some instances, a non-metallic material such as a polymer is placed on the mechanical contact surface of the piezoelectric element in order to fill in surface asperities and to alleviate dimensional tolerances. Such an interposed polymer does not by itself actively place the piezoelectric element in compression, i.e., it does not have an intrinsic urge to do so. Instead, the polymer is itself compressed by the compressive action of the part that supports the piezoelectric element. The compressive action in turn is caused by external means such as, for example, tensioning bolts. Furthermore, the interposed polymer does not completely separate the piezoelectric element from the part holding the piezoelectric element in compression. It merely serves to enhance the mutual mechanical contact. German Patent DE19928780 discloses such an interposed polymer and its uses. Here as well, interposing the polymeric compound and imprinting a mechanical preload are two separate steps.
There is thus a need for a piezoelectric apparatus and manufacturing methods therefore, wherein a piezoelectric element is held in static compression and wherein the compressive force neither is caused by separate mounting hardware such as a preloading screw, nor is due to a separate assembly process such as the press-fit method disclosed in U.S. Pat. No. 6,664,714.